1. Field of the Invention
The present invention relates to a superconductive coil. More particularly, it relates to an improvement of the cooling effect of a superconductive coil.
2. Description of the Prior Art
FIG. 1 is a conventional schematic view of a superconductive coil. In FIG. 1, reference (1) designates a superconductive wire; (2) designates a pancake coil prepared by winding the superconductive wire (1); and (3) designates a cooling channel between the pancake coils (2). The superconductive coil is cooled by a coolant (usually liquid helium). The coolant is fed into the cooling channels (3) to cool the superconductive wire (1).
FIG. 2 is a schematic view of two plates of the pancake coils (2) of the superconductive coil of FIG. 1. Reference (4) is a spacer for forming the cooling channels (3). The cooling channels (3) a width of which is substantially equal to a thickness of the spacer (4) are formed between the pancake coils (2) and the coolant is fed into the cooling channels.
FIG. 3 is a sectional view taken along the line A--A of FIG. 2.
FIG. 4 is an enlarged view of the part of the superconductive wire (1) shown in FIG. 3. Reference (5) is an insulator between turns of the superconductive wires (1). As it is clear from the drawings, the parts of the superconductive wires (1) cooled by the coolant are both side surfaces of the superconductive wires (1). The upper and lower surfaces of the superconductive wires (1) are covered by the insulator (5) between the turns and can not be directly cooled by the coolant.
In the above-mentioned description, it is illustrated that the parts of the superconductive wires (1) cooled by the coolant are both side surfaces of the superconductive wire (1).
The relation of the cooling of the superconductive wire (1) and the current fed to the superconductive wire (1) will be described. Usually, the current fed to the superconductive wires (1) of the large size superconductive coil is decided depending upon the following criterion (full stabilization). Even though the superconductivity of the superconductive wire (1) is broken by certain instantaneous disturbance to result in a resistance of the superconductive wire (1) (normal conductive state), the Joule's heat caused by the superconductive wires (1) is eliminated by the coolant after the elimination of the disturbance. The temperature of the superconductive wire (1) is reduced to less than the critical temperature T.sub.C of the superconductive wires (1) whereby the superconductive characteristics are recovered in the complete stabilization criterion which is shown by the equation: EQU RI.sup.2 .ltoreq.Q(T.sub.C -T.sub.B)S (1)
wherein the reference R designates a resistance of the superconductive wire (1) per unit length in the normal conductive state; I desigates a current fed through the superconductive wires (1); Q(T) designates a heat flux eliminated from the superconductive wires (1) by the coolant; T.sub.C designates a critical temperature of the superconductive wire (1); and S designated a projected area per unit length.
Equation (1) can be changed to equation (2): ##EQU1## The current of the superconductive coil increases depending upon an increase of Q(T.sub.C -T.sub.B) as clearly understood by the equation (2). That is, the current density of the superconductive wires (1) increases. This equation means that an increase occurs in a magnetic field formed by the superconductive coil or also means that it is possible to decrease the length of the superconductive wires (1) at a constant resulting magnetic field. From this viewpoint, it is quite important to increase a heat flux Q(T.sub.C -T.sub.B) eliminated from the superconductive wires (1) by the coolant.
FIG. 5 is an enlarged schematic view of the conventional superconductive wire and B and C designate cooling surfaces.
FIG. 6 is a plane view of a conventional pancake coil (2) winding the superconductive wires (1).
The conventional superconductive coil is formed by plying a plurality of the conventional pancake coils. The cooling surfaces of the conventional superconductive pancake coils are smooth surfaces shown by the references B and D in FIG. 5. The heat flux Q(T.sub.C -T.sub.B) per unit area can not read higher than a constant value.
Therefore, a method of increasing the heat flux Q(T.sub.C -T.sub.B) per unit area by forming many fine grooves (7) which cross in two directions, on the cooling surfaces of the superconductive wires (1) has been proposed as a prior art.
FIG. 7 is an enlarged schematic view of the superconductive wires (1) in the prior art proposed. Many fine grooves having a V shaped sectional view which are mutually crossed are formed on parts of the B and D planes as the cooling surfaces of the superconductive wires (1).
FIG. 8 is a characteristic diagram for comparing the heat transfer characteristic (W/cm.sup.2) per unit projected area of the B (or D) surface on which the fine grooves are formed as shown in FIG. 7 and the heat transfer characteristic of the B (or D) surface which is as smooth as the conventional coil as shown in FIG. 5. In FIG. 8, the heat transfer characteristic on the fine grooves forming surface is shown by the curve (a) and the heat transfer characteristic on the smooth surface is shown by the curve (b). As it is clearly understood, Q.sub.a (T.sub.C -T.sub.B) is about 2.5 times by Q.sub.b (T.sub.C -T.sub.B). The superconductive wires (1) proposed can pass a current of about .sqroot.2.5 (.perspectiveto.1.6) times that of the conventional superconductive wires (1) as shown by the equation (2). The high magnetic field and high current density of the superconductive coil are attained and a compact superconductive coil can be obtained.
The excellent heat transfer characteristic as Q.sub.a (T.sub.C -T.sub.b) shown in FIG. 8 is not always provided by forming the fine grooves in two directions as the B or D surface of FIG. 7. It is therefore necessary to recognize the following condition. That is, the pitch of the fine grooves (7) is 1.5 mm or less in each direction and the depth of the fine grooves (7) is the same or more of the pitch of the fine grooves (7). The superconductive wire having excellent cooling characteristic and a large current capacity can be obtained by forming the fine grooves (7) as shown in the proposed prior art. However, it is a difficult process to form fine grooves in two directions and especially to form crossed fine grooves as shown in the proposed prior art by a cutting or knurling process in the preparation of the superconductive wires though fine grooves in parallel to the superconductive wire can be easily formed.